This invention is related to determining the make-up of copper loops in the public switched telephone network and specifically to determining the make-up of these loops by processing echo signals generated at discontinuities on the loop.
The mainstay of the telephone company local network is the local subscriber loop. The great majority of residential customers, and many business customers, are served by metallic twisted pair cables connected from a local switch in the central office to the subscriber""s telephones. When customers request service, request a change in service, or drop service, these facilities must be appropriately connected or arranged in the field, referred to as the xe2x80x9coutside plant,xe2x80x9d and telephone companies have specially trained craft dedicated full time to this task. Obviously a company needs to have an understanding of its subscriber loops including where they are connected and the location of the flexibility points such as junction boxes, etc. These records historically were kept on paper, called xe2x80x9cplats,xe2x80x9d and more recently are manually entered into a computer database. However, even when entered into a database there are still problems associated with keeping the records accurate and up-to-date.
Having accurate records of the loop plant is critically important to many aspects of a telephone company""s business. In addition to the need for accurate records to provide traditional voice services, there will be a need for even more accurate and detailed records in order to deploy a whole new class of xe2x80x9cxDSLxe2x80x9d based services, including those based on integrated services digital network (ISDN), high-rate digital subscriber line (HDSL), asymmetrical digital subscriber lines (ADSL) and very high rate digital subscriber lines (VDSL) technology. These technologies are engineered to operate over a class of subscriber loops, such as nonloaded loops (18 kft) or Carrier Serving Area (CSA) loops (9 to 12 kft). In fact, the need to be able to xe2x80x9cqualifyxe2x80x9d a loop for provision of one of these technologies is becoming critical, as these technologies emerge and deployment begins. The ability to easily and accurately qualify loops will allow telephone companies to offer a whole range of new services; problems and high expenses associated with qualifying loops can potentially inhibit deployment and/or lower or forego associated new revenues. Unscreened multipair cables in the existing subscriber loop network constitute the main access connection of telephone users to the telephone network. Recently, the demand for new services such as data, image and video has increased tremendously, and telephone companies plan to deliver broadband ISDN services via fiber optic local loops. However, the deployment of fiber optic cables in the access plant will require at least twenty years, so that, in the meantime, it is extremely important to fully exploit the existing copper cable plant.
Although there are many different digital subscriber line services, for example, ISDN basic access, HDSL, ADSL, VDSL, and Synchronous DSL (SDSL), these services are not always available to every customer since copper lines present more problems than expected. In fact, the cable length and the presence of load coils and bridged taps may affect the performance of DSL services. Unfortunately, loop records are unreliable and often don""t match the actual loop configuration, so existing databases cannot be fully exploited.
Loop prequalification is an important issue not only because it can help an economic deployment of DSL services, but also because it can help telephone companies in updating and correcting their loop-plant records. From this point of view, the feasibility of accurate loop make-up identification has much higher economic value than simple DSL qualification.
One way to obtain accurate loop records is to manually examine the existing records and update them if they are missing or inaccurate. This technique is expensive and time consuming. Furthermore, new technologies such as xDSL require additional information that was previously not kept for voice services, so there is the potential that new information needs to be added to existing loop records. Test set manufacturers offer measurement devices that can greatly facilitate this process, but typically they require a remote craft dispatch.
Another way to obtain accurate loop records is by performing a loop prequalification test. There are essentially two ways of carrying out a loop prequalification test: double ended or single-ended measurements. Double-ended measurements allow us to easily estimate the impulse response of a loop by using properly designed probing sequences. Double-ended testing, however, requires equipment at both ends of the loop. Specifically, in addition to equipment at the Central Office (CO) or near end of the loop, double ended testing involves either the presence of a test device at the far end of the loop (Smart Jack or MTU), or dispatching a technician to the subscriber""s location (SL) to install a modem that communicates with the reference modem in the CO. An exemplary double ended system and method that extrapolates voice band information to determine DSL service capability for a subscriber loop is described in Lechleider, et. al., U.S. Pat. No. 6,091,713, entitled xe2x80x9cMethod and System for Estimating the Ability of a Subscriber Loop to Support Broadband Servicesxe2x80x9d (which is assigned to the assignee of the present invention). In addition, craft persons may activate software located at remote sites.
In contrast, single ended tests are less expensive and less time consuming than double-ended tests. Single-ended testing requires test-equipment only at one location, e.g., the CO. In fact, there is no need to dispatch a technician and the CO can perform all the tests in a batch mode, exploiting the metallic access with full-splitting capability on the customer""s line. An example of such a single ended test system is the xe2x80x9cMLTxe2x80x9d (Mechanized Loop Testing) product that is included as part of the widely deployed automated loop testing system originally developed by the Bell System. The MLT system utilizes a metallic test bus and full-splitting metallic access relays on line card electronics. By this means, a given subscriber loop can be taken out of service and routed, metallically, to a centralized test head, where single-ended measurements can be made on the customer""s loop. The test head runs through a battery of tests aimed at maintaining and diagnosing the customer""s narrowband (4 kHz) voice service, e.g., looking for valid termination signatures via application of DC and AC voltages. This system is highly mechanized, highly efficient, and almost universally deployed. In addition, the MLT system is linked to a Line Monitoring Operating System (LMOS) thereby providing a means to access and update loop records which are useful in responding to customer service requests or complaints. However, because this system exclusively focuses on narrowband voice services, the system misses important loop make-up features that will be deleterious to supporting broadband services via DSL technologies.
Another well known single-ended measurement technique relies on the observation of echoes that are produced by medium discontinuities to fully characterize the link. Specifically, these single ended measurements typically rely on time domain reflectometry (TDR). TDR measurements are analogous to radar measurements in terms of the physical principles at work. TDR test systems transmit pulses of energy down the metallic cable being investigated and once these pulses encounter a discontinuity on the cable a portion of the transmitted energy is reflected or echoed back to a receiver on the test system. The elapsed time of arrival of the echo pulse determines its location, while the shape and polarity of the echo pulse(s) provides a signature identifying the type of discontinuity that caused the reflection or echo. Basically, if the reflecting discontinuity causes an increase in impedance, the echo pulse""s polarity is positive; if the reflecting discontinuity causes a decrease in impedance, the echo pulse""s polarity is negative. A bridged tap, for example, produces a negative echo at the location of the tap and a positive echo at the end of the bridged tap. Accordingly, a trained craftsperson is able to determine the type of fault based on the shape, polarity, sequence of pulses.
Nevertheless, TDR methods (or, in general, single ended measurements that rely on echo pulse signatures) are inaccurate and provide ambiguous results that even the most skilled craftsperson cannot interpret. Because the arrival of the echoes is dependent on the location of the discontinuities (or faults) one echo can be masked by another echo if the echoes overlap. In addition, prior art TDR methods do not take into account, more specifically, are unable to separate, the effects of spurious echoes, i.e., echoes generated by a portion of the probing pulse that is reflected from a discontinuity, from real echoes, i.e., echoes generated from the probing pulse being reflected by a discontinuity. Although spurious echoes will be more attenuated than real echoes, they are added to the real echoes causing the real echo signals to be distorted. Accordingly, spurious echoes enhance the ambiguity inherent in TDR measurements because the shape of the echo is used to interpret the type of fault that caused the echo. In other words, a craft person interpreting a TDR measurement analyzes a distorted trace that does not distinguish spurious echo distortion. More importantly, the effects of spurious echoes on the pulse shape cannot be interpreted via human visual inspection. Further, the effective range of today""s commercial TDRs is quite limited since it is impossible to see echoes from discontinuities located more than a few kilofeet away from the detection point. Finally, we are unaware of any commercial TDR having the capability to detect gauge changes.
In Galli, et. al. U.S. application Ser. No. 09/587,459 (hereinafter Galli) a method and system for determining loop makeup based on the echo signatures caused by discontinuities as a pulse traverses a loop are disclosed; note that Galli is also a co-inventor on the present invention. Although the Galli method is able to determine loop makeup more accurately and overcomes the prior art problems highlighted above, the method does have some shortcomings. First, the method works well only where the loop is less than approximately 8,000 feet (8 kft). Once the length of the loop increased significantly beyond 9 kft the method is not able to identify loop discontinuities with the same success because of noise enhancement due to the use of the reciprocal of the insertion loss. Second, Galli""s method may not achieve unambiguous loop make-up identification if the topology of the loop under test does not belong to the set of xe2x80x9cwell-behavedxe2x80x9d loops, i.e., loops that follow the recommended design rules.
Of utility then would be a method and system that overcomes the shortcomings of the prior art, generally, and more specifically the shortcomings of commercial TDRs and Galli.
In accordance with the present invention the limitations and shortcomings of the prior art are overcome by enhancing the range and resolution of commercial TDRs and by allowing accurate unambiguous determination of a subscriber loop make-up.
In particular, an aspect of the present invention is a method and circuitry for enhancing the performance of commercial TDRs. In more detail, the inductive effect of a subscriber loop is taken into account when processing echo signals that are the result of probing the loop with pulses. In accordance with this aspect of our invention we remove a slowly decaying signal caused by the inductive effect of the loop from the echo signals. Accordingly, the echo signals are no longer masked by the slowly decaying signal thereby increasing the accuracy and range of a measurement system built in accordance with the present invention. This particular aspect of the present invention will be useful in TDR measurements by increasing their range and by allowing gauge change detection. The effective range of a TDR designed in accordance with this aspect of the present invention is dependent only on the energy of the probing pulse, i.e., in principle the range is unbounded. In accordance with this aspect of our invention, differential probing circuitry that improves the performance of TDRs is disclosed. The probing circuitry improves the accuracy of a TDR by rejecting the deleterious effects of common mode propagation.
In another aspect of the present invention, we provide a method for unambiguous and precise loop make-up identification. In accordance with this aspect of the present invention the input impedance of the loop as a function of frequency is used in the process of identifying discontinuities and other features represented by the echo signals. This is accomplished by first calculating the input impedance of the loop as a function of frequency. The input impedance of the loop is then convolved, in the frequency domain, with the Fourier transform of the probing signal. Finally, a simulated waveform of the discontinuity in the time domain is obtained by inverse Fourier transforming the result of the convolution. This simulated waveform is then compared to the actual echo signal caused by the discontinuity. If the comparison yields an acceptable match, e.g., within a predetermined error margin, then the discontinuity is identified and the signal corresponding to that discontinuity is removed by subtracting the simulated waveform from the acquired data. This is done for each discontinuity encountered until the last discontinuity is identified. In accordance with this aspect of the invention all the shortcomings of Galli are overcome.
By employing all the foregoing aspects of the present invention, it is possible to completely determine the make-up of a loop of any length and any topology.